Incorrect. The diameter of the idler wheel will have absolutely no effect on the final drive speed. The only way to change the final drive speed (short of changing the motor speed) is to change the diameter of the driving member (motor pulley) or the driven member (platter itself). The idler pulley simply transfers the energy from the driving to the driven.

Incorrect. The diameter of the idler wheel will have absolutely no effect on the final drive speed. The only way to change the final drive speed (short of changing the motor speed) is to change the diameter of the driving member (motor pulley) or the driven member (platter itself). The idler pulley simply transfers the energy from the driving to the driven.

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I stand corrected, sorry for any confusion. In my mind I was thinking about the drive wheel which turns the platter versus the idler (intermediate between motor and drive wheel).

There are some idlers which are actually "double decker" wheels where a larger wheel is driven by the motor pulley and a smaller wheel drives the platter.

Reducing the size of either of those wheels WILL reduce the speed of the platter.

EDIT: Oops. I have to correct this. If the larger wheel is reduced (the one driven by the motor pulley) the platter speed will increase. If the wheel driving the platter is reduced, platter speed will decrease.

My Presto.T18-turned-into-a-Lenco project will speed up or slow down depending on which Lenco wheel I have installed. The only difference is an old worn wheel vs. a new Mirko wheel. This is confirmed by a KAB Speed Strobe mat and light. It requires a speed readjustment if I change wheels (easily done with my DC motor and its controller board).
The drive capstan on this deck is one fixed size, not stepped or tapered, and speed changes occur by slow and speeding the motor to appropriate drive speeds. With that said, there is an obvious speed difference in just changing the tire size; the Mirko wheel is cut to the diameter of a new Lenco wheel.

I've experienced speed changes with old vs. new idlers before too, but it wasn't because of the diameter. The new wheel gripped properly and corrected the speed inconsistency whereas the old wheel caused the speed to go off. Putting the old wheel in a drill press and gently scratching off a little of the old surface with crocus cloth put it right again.

The aluminum Lenco wheels stay soft, even after 40 years; fine rubber.
By "worn" I'm merely suggesting that the knife edge of the wheel had worn down to a rounded knife edge; minor variance, but noticeable speed change.

How is it not debatable? - I experienced it extensively in the driveline renovation and redesign, dry-fitting, and final execution of my renovated deck. I'm not extending hearsay, rather, I'm extending firsthand facts of what happened over a years retooling of an idler deck.

We need to see some hard equations here. It's not a debatable subject.

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1 = 1, INPUT SPEED = OUTPUT SPEED, Rotational velocity both receives and sends at the same speed; conservation of energy, energy can be neither created nor destroyed without the expenditure or expulsion of force.

I've understood since I was a 5 year old kid playing 78s that idlers were speed neutral, they neither add nor subract rotational velocity. The rim would have to shrink or expand during a half-turn, with the change in mass being created from or sent to a fifth dimension. It can't happen.

The angle of the two contact points remains fixed. It can't change, it is hard-coded into the construction of the wheel. The angle of bisection would have to change during rotation for there to be a speed differential.

P.S. I have several Lencos, an idler CAN affect speed if the ANGLE between motor pulley and platter changes. If the receiving point remains at the same place on the motor pulley, but the drive point is moved INWARD on the platter, then the platters's driven radius becomes smaller and the platter turns faster. That's not the SIZE of the idler, but how the lever holding the idler is bent.

On a Lenco the idler describes an arc UNDER the platter, if the EDGE of the platter is driven then even a skewed idler does not affect speed (except via edge crawl)

The Lencos are unusual in that they have a cone shaped pulley for that infinitely variable speed, and wear in the idler changes where it contacts the pulley, so that it does change speed, but because the pulley contact diameter has changed. I suppose this might also happen with more normal idler that have speed adjustments. But all that's happening is that the idler is transmitting a higher linear velocity (or do I mean angular -- it's late in the day and my mind isn't as sort of sharp as it can be).

Nat,
I fit a Lenco wheel into a Presto T18, converting it from a three wheel Presto idler, to a single wheel idler, driven by a capstan without steps or tapered capstan. Its now an internal rimdrive Presto driven by a Lenco wheel, and precision DC motor w/ controller card, not a Lenco deck.
This is a side view of the redesigned driveline:
The driving portion of the capstan is not tapered. The only taper in the capstan was to allow the captan to run closer to the decks top plate, but its below the drive portion.
From this build:http://www.audiokarma.org/forums/showthread.php?t=380100